Accumulated Other Comprehensive Income (Schedule of accumulated other comprehensive income (loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Amounts reclassified from accumulated other comprehensive income	$ 3,831		$ 8,176
Commodity Derivatives [Member]
Amount of Gain or (Loss) Recognized in Income	(1,349)		(1,993)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	2,482		6,183
Tax expense	(968)		(2,396)
Total reclassification for the period	1,514		3,787
Oil And Natural Gas Revenue [Member] | Commodity Derivatives [Member]
Amounts reclassified from accumulated other comprehensive income	3,831	[1]	8,176	[1]
Amount of Gain or (Loss) Recognized in Income	0		0
Gain (Loss) on Derivatives [Member] | Commodity Derivatives [Member]
Amounts reclassified from accumulated other comprehensive income	0		0
Amount of Gain or (Loss) Recognized in Income	$ (1,349)	[2]	$ (1,993)	[2]

[1] Effective portion of gain (loss).

[2] Ineffective portion of gain (loss).